In the 20th century,
movies and television have contributed substantially
to the transmission of social and cultural values, because of the
power that these media have to evoke reality. For a long
time, scientists thought that the phenomenon of retinal
persistence explained why we experience the succession of still images in a film as
if they were scenes that are actually moving. Retinal persistence
lets us store a visual impression in memory for a few
hundred milliseconds after the visual stimulus has disappeared,
so scientists inferred that this phenomenon might fill in
the intervals of darkness between the still images that are
projected rapidly when we view a film. In this way, it was
thought, each new image was imprinted on the retina before
the impression of the preceding one had faded completely,
so that the one dissolved into the other, creating the illusion
of continuous movement.
Eventually, however, psychologists
rejected this explanation, for several reasons. For one thing,
we continue to have the illusion of movement even when still
images are presented to us at speeds of 10 per second or
even slower.
One of the other problems pointed out was that if retinal persistence
actually played such a significant role in creating the illusion
of movement, it would do so by piling new images on top of old
ones that were still discernible. The differences in the positions
of the objects in the images would therefore create trails like
those you see in time-lapse photographs that show all the components
of a series of movements simultaneously.
A yet greater problem for this theory was that retinal persistence appears only
about 50 milliseconds after the image ceases. But when a film is projected at normal speed,
the viewer sees at least two still images during this interval. Consequently,
the first image would not start to "persist"
until the second one was already being projecteda serious
contradiction to the theory that the illusion of movement
is created when the first persistent image blends into the
second.
The illusion of movement in motion pictures is now believed to produced by a
different phenomenon, known as the beta effect. This effect occurs when two images
whose elements are in slightly different positions from each other are presented
one after the other in quick succession. The brain then automatically perceives
movement, because of the way that the receiving fields of the retinal cells and
the various areas in the visual cortex integrate
visual information to detect movement and determine its direction.
So in a sense, we are victims of the beta
effect whenever a series of still images passes in front of our
eyes rapidly, whether we are watching a dramatic film, or a documentary,
or a cartoon, or even looking at one of those little books where
the images seem to move when you flip the pages quickly (see box
below).
There is another theory that retinal persistence does play a role in the way
we perceive motion pictures, by reducing the amount of flicker that we notice
as the projector shutter opens and closes 48 times per second. But even this theory has been called into question.
Retinal persistence actually involves
two distinct phenomena. The first, positive afterimage,
is the one that was long cited to explain how motion pictures
work. One good example of a positive afterimage is the
image of a flash that persists in your vision for a fraction
of a second after someone takes your photo with a flash
camera.
The second phenomenon, negative afterimage,
can last for several seconds after the stimulus stops, so
it is easier to perceive. The reason that it is called a
negative afterimage is that its elements have the complementary
colours of those in the original image, and the opposite
luminance. For example, if you stare at a green object
for a while, you will see a red image of this same object
for a few seconds if you quickly look away from
it to a white surface.
Flipbooks are a very simple form
of animated film that illustrates the beta effect. In these
small books, each page contains a drawing that is slightly
different from the one before it. When you flips the pages
rapidly, your eye sees each image for a brief instant and
interprets the series of changes from one image to the
next as movement.